A Polybenzimidazole (PBI) membrane impregnated with phosphoric acid is a state of art of high temperature polymer electrolyte membrane for fuel cell (HT-PEMFC). Conventional PBI is not simple to prepare and involves immense economic inputs. Moreover, its monomer is a known carcinogen. Among the family of PBIs, ABPBI is one of the best choices to be applicable as a membrane material in HT-PEMFC. However, less attention has been paid to this polymer due to intrinsic difficulties.
Conventional ABPBI is soluble in very few solvents like strong acids and thus difficult for membrane preparation (due to the corrosive nature and high boiling points of these acids required during casting), refer Romero P. G. et al, Fuel cell 05 (2005) 336).
Conventionally ABPBI is synthesized from 3,4-diaminobenzoic acid (DABA) in polyphosphoric acid (PPA) as a solvent.
In most of the literature on ABPBI, membranes are prepared by solution casting method (by evaporation of methane sulphonic acid, MSA at ˜200° C.). ABPBI based membranes can also be prepared by direct acid casting of MSA/ABPBI/H3PO4 solution, refer Romero P. G. et al., JMS 241 (2004) 89; in which the polymer solution casted on the substrate is dipped into H3PO4.
Recently J. A. Asensio et al. in Fuel cells 05 (2005) 336 discloses process for synthesis of ABPBI or other PBIs by self-condensation of 3,4-diaminobenzoic acid in MSA/P2O5.
Synthesis of Poly (2,2′-(1,4-phenylene) 5,5′-bibenzimidazole) (para-PBI) and Phosphoric Acid Doped Membrane for Fuel Cells is reported in Fuel cells (2009), 09318 by S. Yu.
Phosphoric acid impregnated ABPBI membranes were cast by Cho J. et al., from an ethanol/NaOH solution (Cho J. et al., JPS B 42 (2004) 2576). In this process, ABPBI needs to be dissolved in the alcoholic solution of caustic and then casting into a membrane is done. These membranes need to be doped with phosphoric acid for their use as membranes for fuel cell. Kim et al., prepared the ABPBI membranes from polymerized solution containing CH3SO3H and P2O5 casted on glass plate, immersed in water bath and later dried under vacuum (Kim H. J. et al., Macro. Mol. rapid comm. 25 (2004) 894).
ABBPI copolymer with isophthalic acid based polymer was prepared by Ronghuan He's group. Membranes were prepared using DMAc as a solvent by solution casting method (He R. et al., Poly. Int. 59 (2010) 1695).
ABPBI copolymer with terephthalic acid was synthesized and membranes were prepared from MSA solution using casting method (Lee J. C. et al., Macromol, Mrtl. Engg. 296 (2008) 914). Both these methods involve multiple steps; viz.; (i) polymer synthesis (ii) its isolation (iii) dissolution in solvent (iv) membrane casting and then (v) doping with H3PO4.
There is no literature where ABPBI reaction mixture after its synthesis is used directly to cast the membranes by sol-gel method. Further, methanesulphonic acid (MSA) is required as a solvent for membrane to be formed by solution casting method, which involves evaporation of corrosive solvent. When polymerization solvent is MSA, often P2O5 or polyphosphoric acids are also used along with MSA. Though polymerization of ABPBI is known using polyphosphoric acid (PPA) as a solvent, membrane casting by conventional sol-gel process is not reported. When the inventors attempted the same, rather than obtaining a membrane (a film), the polymer phased out in the form of powder, as shown in FIG. 1. Hence preparation of ABPBI based membranes using its solution immediately after its synthesis and possessing good mechanical strength as well as acid content tuneability is a need in the art.
In the light of above, the inventors have developed rigid aromatic moiety incorporated co-ABPBI and membrane thereof in the film form by employing sol-gel method, that obviate the cumbersome and lengthy process steps described in the prior art.